1. Field of the Invention
The invention relates to the fabrication of electric lamps, and in particular to the fabrication of capsules for use in filamented lamps. Although the invention is applicable to filamented lamp tubes of any size, it is particularly useful to relatively small, low wattage lamps.
2. Description of the Prior Art
Although relatively expensive to manufacture, small volume tungsten halogen filamented lamps are gaining commercial interest due to their superior light output and energy efficiency, especially when coated with an optically reflective coating. Typically, the fabrication of a double ended, filamented capsule for a tungsten halogen lamp includes a series of separate steps. For example, one known method includes the following steps: (1) Starting with a quartz tube an appropriate length, one end is heat sealed. (2) The tube is then loaded in a lathe chuck system that may be vertical or horizontal oriented. (3) The tube is rotated in the lathe and an appropriate length is heated to its working temperature. (4) The molten glass is captured in a two-part mold and nitrogen is blown into the tube through the open end. The positive pressure causes the heated glass tube to fill the mold to form a shaped capsule. (5) The shaped capsule is trimmed to length. (6) An exhaust tube is sealed onto the body of the capsule. (7) The capsule is acid-washed, dried and vacuum-baked to reduce possible contamination. (8) A filament assembly is loaded into the capsule. (9) The filament assembly is sealed in the capsule. (10) The capsule is then exhausted and filled with a halogen lamp fill through the exhaust tube. (11) The lamp capsule is finally tipped off to seal in the lamp fill.
As is readily apparent from the number of individual steps involved, the cost of manufacturing a small filamented lamp capsule may be relatively high. There are numerous chances for contaminants to be introduced into the process, hence the need for the washing, drying and baking steps during the course of fabrication.
Use of tungsten halogen lamps in areas requiring high reliability, and miniaturization, such as the auto industry, has intensified the need to avoid the long and costly manufacturing process. A more uniformly shaped volume and a tipless lamp capsule may be preferred features to avoid degradation of the lamp's optical qualities, and in particular where the lamp capsule is to receive an infrared reflective coating on the capsule surface. All of these features are particularly difficult to achieve in small filamented lamps, for example those having a volume less than two milliliters.
There is a need for a fabrication technique which reduces costs by simplifying the manufacturing process, and still address the specific problems encountered in low wattage lamps. There is also a need to provide a fabrication technique which provides a means of positioning filaments in an capsule in a repeatable manner. It may also be desirable to provide a lamp capsule having a uniformly shaped volume and one which is tipless. It is also desirable to achieve these features in a small volume filamented lamp.